169 years ago today, July 9, 1856 (June 8, 1856 in the lunar calendar), the famous chemist Avogadro passed away. Ameldeo Avogardo (1776 - 1856) was an Italian physicist and chemist. Born on August 9, 1776 into a noble family in Turin. On August 9, 1792, he entered the University of Turin to study law. In 1796, he obtained his doctorate in law and later worked as a lawyer. From 1800 to 1805, he studied mathematics and physics, and then mainly engaged in physics and chemistry. In 1811, the theory of molecules was proposed: molecules are composed of atoms. It is deduced that the same volume of gas contains the same number of molecules at the same temperature and pressure, also known as Avogadro's law. In 1803 he published his first scientific paper. In 1809, he was appointed Professor of Natural Philosophy at Welcelli College. In 1811, he was elected as a member of the Turin Academy of Sciences. Avogadro devoted his life to the study of atom-molecular theory. In 1811, he published a paper entitled "Method for Determining the Relative Mass of Atoms and Determination of the Ratio of Numbers When Atoms Enter a Compound." Based on Guy Lussac's gas compound volume ratio experiment, he made reasonable assumptions and reasoning. First, he introduced the concept of "molecule" and distinguished it from the concept of atom, pointing out that atoms are the smallest particles participating in chemical reactions, and molecules are the smallest particles that can exist independently. Molecules of a simple substance are composed of atoms of the same element, while molecules of a compound are composed of atoms of different elements. The article clearly states: "It must be admitted that there is also a very simple relationship between the volume of gaseous substances and the number of simple or complex molecules that make up gaseous substances. The one, and even the only allowable assumption that connects them is that all gases in the same volume have the same number of molecules." In this way, the measurement of the atomic weight, molecular weight and molecular composition of the gas can be completely consistent with the laws obtained in physics and chemistry. Avogadro's hypothesis was later called Avogadro's law. Avogadro also studied in detail the method of determining molecular and atomic weights based on his law, but his method was not accepted by people for a long time. This was because the scientific community could not distinguish between molecules and atoms at that time, and the molecular hypothesis was difficult to understand. In addition, chemical authorities at that time refused to accept the molecular hypothesis, resulting in his hypothesis being shelved in obscurity for half a century. This is undoubtedly a major regret in the history of science. Until 1860, the Italian chemist Canizaro made a vigorous speech at an international chemical conference, claiming that his native Avogadro had solved the problem of determining atomic weight half a century ago. With sufficient arguments, clear organization, and easy-to-understand methods, Cannizaro quickly convinced most chemists that Ahuagadro's theory was universally correct. But by this time Avogadro had died silently a few years ago and had not been able to see with his own eyes the victory of his doctrine. Avogadro was the first to realize that matter is made of molecules, and molecules are made of atoms. His molecular hypothesis laid the foundation for the atom-molecule theory, promoted the development of physics and chemistry, and had a profound impact on modern science. His four-volume book "The Physics of Weighing Objects"(1837 - 1841) was the first course on molecular physics. Avogadro died in Turin on July 9, 1856.